This invention is directed to a battery charging circuit wherein a solar energy source is utilized to charge same, and in particular to a circuit for detecting a charged condition of the battery in order to prevent overcharging of same.
In recent years, solar batteries have gained wide popularity in view of their reliability, longevity and low price. In view of the benefits obtained therefrom, such solar batteries have been utilized in unmanned relay stations, lighthouses, sea lighting buoyies and rain and water gauge telemetering equipment.
Similarly, for general consumer use, solar batteries have been utilized as the sole power source for portable radios, electronic timepieces and the like. Accordingly, the number of solar batteries in use has rapidly increased as the increase in uses therefor have been found.
Solar batteries generate electricity by irradiation of the sun's rays, thereby rendering it necessary to use same in combination with a secondary battery in order to continuously apply electric power to the load. The most frequent use of a secondary battery is a Ni-Cd Alkali storage battery since same is very endurable, particularly with respect to overcharging and excess discharging. Moreover, Ni-Cd Alkali storage batteries can be charged by a small amount of current.
In order to utilize the solar battery as a source of charging voltage and current, the amount of voltage and current is obtained by connecting a group of solar battery elements in series or in parallel as required. Thus, the current or voltage required is obtained by making the proper connections. Nevertheless, with Ni-Cd secondary batteries, two different types of batteries are available, open batteries and hermetically sealed batteries. As to the former, a charger is utilized because of the limited charging capability of the battery. This is particularly true since the charge must be instantaneously applied if the secondary battery is to be properly charged, since a charge is always needed. Accordingly, a charging circuit is used in connection with a dry battery and a storage battery of the hermetically sealed type.
However, conventional storage batteries cannot be sealed hermetically because oxygen is produced at the positive electrode and hydrogen at the negative electrode as the battery reaches a completely charged state. Nevertheless, if a means is provided for effecting oxygen consumption by reaction at either the negative electrode, or at an additional electrode, it is possible to stably charge a Ni-Cd battery of the hermetically sealed type. Nevertheless, even in such a battery construction, when the charging current is excessive and the battery is overcharged thereby, an inbalance between the oxygen generating process and the oxygen consuming process is effected, causing an excess generation of gas, and hence the risk of causing a gas compression explosion.
In view of the risks of explosion caused thereby, charging currents are usually limited to 0.1 C and less in order to maintain the balanced condition between generation and consumption of oxygen. Nevertheless, when a charging circuit maintains such a low charging current, a considerable amount of current is wasted when the solar source senses high intensity light. Additionally, an excessive amount of time is required to charge the secondary battery. Accordingly, a solar source charging circuit adapted to effect an immediate charging of a secondary battery and yet being adapted to avoid overcharging thereof is desired.